1. Field of the Invention
The present invention relates generally to the field of semiconductor lasers, and more particularly, the present invention relates to a semiconductor laser suitable for use as a light source of an optical disc device, for example.
2. Description of the Related Art
One known semiconductor laser having a design of a stabilized transverse mode is a buried-ridge semiconductor laser having a striped structure. FIG. 10 is a perspective view of a conventional AlGaInP-based buried-ridge semiconductor laser having a straight striped structure.
As shown in FIG. 10, the AlGaInP-based buried-ridge semiconductor laser having a straight striped structure is formed by sequentially stacking an n-type AlGaInP cladding layer 102, a GaInP active layer 103, a p-type AlGaInP cladding layer 104, a p-type GaInP intermediate layer 105 and a p-type GaAs contact layer 106 on an n-type GaAs substrate 101.
An upper-layer portion of the p-type AlGaInP cladding layer 104, the p-type GaInP intermediate layer 105 and the p-type GaAs contact layer 106 form a straight ridge stripe extending in one direction. Numeral 107 denotes a ridge stripe portion made of the upper-layer portion of the p-type AlGaInP 104, the p-type GaInP intermediate layer 105 and the p-type GaAs contact layer 106. The straight ridge stripe portion 107 has a uniform width W' in the cavity lengthwise direction. The width W' of the ridge stripe portion 107 is the width of the bottom of the ridge stripe portion 107. N-type GaAs current blocking layers 108 are buried in opposite sides of the ridge stripe portion 107 to form a current blocking structure.
The laser includes a p-side electrode 109, such as, for example, a Ti/Pt/Au electrode, formed on the p-type GaAs contact layer 106 and the n-type GaAs current blocking layer 108. An n-side electrode 110, such as, for example, a AuGe/Ni/Au electrode is formed on the bottom surface of the n-type GaAs substrate 101.
The conventional AlGaInP-based buried-ridge semiconductor laser selects the width W' of the ridge stripe portion 107 to be 5 .mu.m or less in order to stabilize the transverse mode.
Additionally the semiconductor laser controls its guide mechanism in accordance with thickness d' of the p-type AlGaInP cladding layer 104 at opposite sides of the ridge stripe portion 107. More specifically, the guide mechanism of the buried-ridge semiconductor laser is real index-guided when the thickness d' of the p-type AlGAInP cladding layer 104 is 100 to 300 nm, intermediate between real index-guided and gain-guided natures when the thickness d' is 300 to 500 nm, and gain-guided when the thickness d' is 500 nm or more.
When the guide mechanism of the conventional buried-ridge semiconductor laser is real index-guided, the transverse mode is confined by a step in index of refraction formed in parallel to the junction. When it is gain-guided, the transverse mode is confined due to a gain distribution caused by distribution of injected carriers.
It is known that self-pulsation occurs when the guide mechanism is intermediate between real index-guided and gain-guided natures. In this case, although the transverse mode is confined by a step in refractive index made in parallel with the junction, the step in refractive index is smaller than that of the real index-guided structure, and extension of light in parallel with the junction is larger than that of the real index-guided structure. Therefore, as shown in FIG. 11, the width W.sub.p ' of the light confinement region becomes larger than the width W.sub.g ' of the gain region in the GaInP active layer 103. As a result, a saturable absorber 111 is produced in the GaInP active layer 103 at opposite sides of the ridge stripe portion 107 due to the difference between the light confinement region and the gain region.
However, the conventional buried-ridge semiconductor laser having a straight stripe structure has the following problems. Specifically, the width W, of the ridge stripe portion 107 having a straight shape must be 5 .mu.m or less in order to maintain a stable transverse mode. In this case, as the contact area of the p-type GaAs contact layer 106 and the p-side electrode 109 decreases, the current path is narrowed, and the differential resistance increases, which causes an increase in required driving voltage of the semiconductor laser.
When the conventional buried-ridge semiconductor laser is used as a light source of an optical disc device, for example, it is effective to minimize the spot of laser light on the emitting end surface and to enlarge the horizontal radiation angle .crclbar.// in the far field pattern to 8.degree. or more approximately. For this purpose, the width W' of the ridge stripe portion 107 must be narrower. In this case, however, the gain region in the GaInP active layer 103 becomes narrow, and distribution of light to regions with a high absorption coefficient becomes larger. Therefore, the guide loss increases, and the drive current of the semiconductor laser increases. This is important when the conventional buried-ridge semiconductor laser has a real index-guided structure liable to decrease the radiation angle .crclbar.//. If the conventional buried-ridge semiconductor laser has a gain-guided structure, the far field pattern of the laser light appears as double lobes, and may cause problems for practical use.
When the conventional buried-ridge semiconductor laser is used as a light source of an optical disc device, for example, it is effective to configure the conventional buried-ridge semiconductor laser for self-pulsation in order to reduce the noise. In this case, however, since the range of allowable values of laser structure parameters (for example, thickness d' of the p-type AlGaInP cladding layer 104 at opposite sides of the ridge stripe portion 107) is very small, the yield is low, and it is difficult to realize a self-pulsation semiconductor layer. Additionally, since the saturable absorber 111 generated by a difference between the gain region and the light confinement region of the GaInP active layer 103 is unstable with changes in temperature and optical output during operation, self-pulsation is unstable.